Electron cyclotron resonance (ECR) systems, also referred to as ECR tools, have been widely investigated and are increasingly being used for high speed processing of samples such as semiconductor substrates. For example, ECR tools are typically used to etch a semiconductor substrate or to deposit thin films of materials on such a substrate.
As is well known to those having skill in the art, an ECR tool typically includes an airtight processing chamber and a microwave source for generating microwave energy and applying the microwave energy to the chamber. A magnetic field is also generated in the processing chamber. A gas is supplied in the processing chamber, with the particular gas depending upon the particular operation to be performed. The frequency of the microwaves and the magnetic field strength are chosen to create electron cyclotron resonance in the electrons in the gas, to thereby generate a plasma of the gas in the processing chamber. The plasma generates from the gas the reactive species which are used to perform etching or deposition according to well known techniques.
ECR tools possess a number of advantages compared to conventional deposition and etch tools. For example, ECR tools provide electrodeless operation which reduces chamber wall contamination. The low plasma potential operation provided by ECR tools reduces ion induced surface damage to the substrate itself. Moreover, high plasma density and low pressure operation can produce high deposition and etch rates with low particulate formation. These advantages have become increasingly important for processing state of the art integrated circuits as device features shrink to submicron sizes. The general design and operation of ECR tools is described in U.S. Pat. No. 4,859,908 to Yoshida et al. entitled Plasma Processing Apparatus for Large Area Ion Irradiation; U.S. Pat. No. 4,727,293 to Asmussen et al. entitled Plasma Generating Apparatus Using Magnets and Methods; U.S. Pat. No. 4,450,031 to Ono et al. entitled Ion Shower Apparatus; U.S. Pat. No. 4,417,178 to Geller et al. entitled Process and Apparatus For Producing Highly Charged Large Ions and an Application Utilizing This Process, and Japanese published patent application 88-310887/44 entitled Film Forming Plasma-Generating Machine.
Present efforts to improve ECR tools have focused on increasing the uniformity of the plasma generated in the tool and in increasing the overall efficiency of the tool. Increased uniformity is necessary to ensure uniform deposition or etching conditions across the surface of the substrate to be processed. As the size of substrates (wafers) increases, the need for uniformity becomes more critical. Increased efficiency is necessary so that high density plasmas can be generated using minimal microwave and magnetic power to thereby increase processing throughput. Efficiency increases may include, among other things, simplified operation, increased reliability, decreased need for attended operation, and decreased size.
One attempt at increasing the uniformity of the plasma is described in European Patent Application 87/311,451.6 to Nakamura et al. entitled Plasma Apparatus. Described is an ECR tool having a second magnet in addition to the main magnet, with the second magnet functioning to more evenly distribute the flux density adjacent the substrate to be processed. Another attempt to improve the plasma azimuthal uniformity is described in U.S. Pat. No. 4,877,509 to Ogawa et al. entitled Semiconductor Wafer Treating Apparatus Utilizing a Plasma. Disclosed is a rectangular microwave waveguide having a rectangular-to-circular microwave converter and a circular polarization converter for transforming the circular TE.sub.11 mode microwave supplied from the rectangular to circular microwave converter into a circularly polarized wave. By using a circular polarized TE.sub.11 wave, the electric field strength of the microwave supplied to the chamber is azimuthally symmetric when averaged over time to thereby make the density of the plasma generation uniform. Improved etching is thereby provided.
As described above, other attempts to enhance the performance of ECR tools have concentrated on the efficiency of the microwave source itself. For example, U.S. Pat. No. 4,788,473 to Mori et al. entitled Plasma Generating Device with Stepped Waveguide Transition describes a microwave source having a rectangular waveguide which is connected to a vacuum sealing dielectric window, with the waveguide tapering in the direction of microwave propagation. The taper is provided by a series of decreasing size waveguides, having quarter wavelength and half wavelength length. The tapering waveguides allow the use of a smaller magnetic coil for generating the magnetic field thereby decreasing the size and weight of the magnetic coil and improving efficiency.
Other attempts to increase ECR efficiency have used external tuners to optimally tune the incoming microwave energy and eliminate unwanted reflection. See for example U.S. Pat. No. 4,507,588 to Asmussen et al. entitled Ion Generating Apparatus and Method for the Use Thereof. As disclosed, the microwave source coupler is tuned by means of a "sliding short" during operation. This external tuning is helpful to reduce reflection of the microwaves.
Unfortunately, the use of external tuning is undesirable in ECR tools for many reasons. First, external tuning must typically be manually performed, and readjustment is typically required for each new plasma condition. In fact, readjustment is often required even if plasma conditions are unchanged, because of unavoidable changes in the process. Moreover, it is often difficult to tune the microwave source at all due to the number of modes which are being launched by the microwave source. Accordingly, often the plasma jumps between modes and hysteresis can occur in the tuning. The need for external tuners makes ECR tool operation and process development less repeatable and also prevents the achievement of optimum plasma densities, thereby reducing the available processing rate.